Re: plip with mulinux

From: Sven Conrad (
Date: Mon Sep 04 2000 - 21:31:50 CEST

Michele Andreoli wrote:

> > me in mind.
> Yes, of course!

What a honor :-))) Thanx.

> I need exactly of topic 2): I have two ethernet card, plip and ppp
> working. I can link two PC and soon a 3th PC. I can experiment.
A 3. PC is the minimun to get rid of pc-pc link only setups. Then
you can try to setup e.g. pc1<-plip->pc2<-eth->pc3<-ppp->internet.
I think this will be a good expirience. I can help, whenever you
need (or who ever what's to join such experiments)

> >
> > I can't see mulinux computers as major gateways in Company or
> > Universities.
> > So what I see is, mulinix as Internet Gateway/Firewall at home, as
> > Playground for workgroops or things like this.
> I wish not setup something for a particula purpose, but only use
> the little system in the way I can learn something, no problems.

what I ment is the strange fact, that the 'normal' and so 'easy'
setup is the most unusual for mulinux. This implies that the
setup needs to be more general and allows what the user likes to
> > So far ok. But that is mostly not sufficient, because you have only the
> > chance to alter the mulinux box. But if you want more than point to
> > point
> > connects, then EVERY box in the network needs all gateway routes! If
> > this
> > is, what you want, go ahead and set gateway routes for every subnet you
> > have and you are all set. (this depends totaly on the structure of the
> > entire network. E.g. gateways can be in a daisy chain!)
> If you means that any box has to set the Linux box as gateway, I
> can understand.

Yes that is it. "Gateway" is not a special setup or hardware. The
itself is the only one, who don't know, that he is one! (cruical?)
A gateway is only a box, which normaly has access to more than one
network and so, can deliver packets, that others don't. But e.g. for
a TCP connection, both boxes need to use the gateway for be able to
have a bidirectional conversation. So both boxes have to have this
gateway routes.

For confuse you a liddle bit: ;-)
You have 3 pc with each one eth interface. This means a normal small
You setup on pc1 and pc2 only a route to the interface,
and the 'normal' LAN to reach over pc3 as gateway. Then all is working.
pc3 has the usual setup and knows nothing about being a gateway.
Every packet between pc and pc2 is 2 times on the bus, first on the way
to pc 3 and a second time to the real destination.

> More gateways? Please, explain better: how a single gateway is choosen?
> Based on the network mask? On gateway max for interface or more?
Yes. Think about my 3 pc network above. There you have 3 subnetworks.
Because they are in line, all is going through one interface. If you
want to be exact you use routes for every subnet, even if there can
be redundant routes, because default routes cover allso routes with
a more restricted mask.
But in generell, I would like to say, there is one route on EVERY box
for EVERY subnet in the entire network. The question is only, is the
subnet connected to a local interface or is it reachable over a
gateway(which needs to be in a reachable subnet).
Please think about the internet as a subnet, even if it is a liddle
bit bigger.
(for the table seek of route, look into the second mail)

> > Here we are at a dificult question. Every interface needs at least one
> > route,
> > but can have more!
> > e.g:
> > [eth0 =]
> > route add -net netmask
> > route add -net netmask gw
> > route add -net netmask gw
> >
> > which means, this box is a gateway to a "backbone" with two other
> > gateways
> > and there subnets behind! All these routes go to eth0!!! (route sees
> > this
> > allone, because the gw's are in the 1 net)
> Obscure thing :-( I'm predisposed to considere any interface connected
> to only a network. I this example, I have to considere the eth0
> connected to 192.168.*.* ? If yes, I can't understand netmask used.
Nope. eth0 could only have /24 netmask. Look at the 1. route. This route
say, that the subnet is direct connected to eth0. The
subnet's 2 & 3 are not connected! So they are only reachable via
The gateways itself lay on the subnet 1 so they are reachable.

[box] --(subnet1)------- [gw1] ----- (subnet 2)
            !------------[gw2]------ (subnet 3)

> >
> > BTW: could a cat /proc/net/dev be easyer to parse than the output of
> > ifconfig?
> They do not seems totally equivalents: ifconfig shows all active
> interfaces. In /proc/net/dev I found all *configured* interfaces.
> I found also interfaces currently down in this file!
> Please, do you know also the bridging topic?

I'am not too familiar with these vocabulars. So I first have to look
into some doc/howto.

At this level, there are bridges and routers, and below repeaters and
bubs. But when I remenber correct, this reflect more hardware issues.
AFAIK we have used a bridge to the internet. Has been a black box,
allows NAT, firewalling and stuff like this. But I may be compleatly
wrong. So please wait until you get more prooved infos.


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